Device for vaporizing and/or superheating a combustible

ABSTRACT

A method for vaporizing and/or superheating a combustible/water mixture, wherein the combustible is especially methanol, for supplying a gas generation system ( 2 ) belonging to a fuel cell installation ( 3 ). The waste-gas from the fuel cell and/or gas generation system is catalytically combusted together with a gas containing oxygen in order to produce the thermal energy required therefore. The combustible is added in a dosed manner to the volume flow from the gas containing oxygen and the waste gases of the fuel cell and/or gas generator system in the direction of flow prior to catalytic combustion.

BACKGROUND OF THE INVENTION

The present invention relates to a method for vaporizing and/orsuperheating a fuel or a fuel/water mixture for a fuel cell system.Moreover, the present invention relates to a device for carrying outsuch a method.

Heat exchangers which are suitable for vaporizing and/or superheatingmedia are known from the related art. Thus, for instance, German PatentDE 44 26 692 C1 describes a heat exchanger which is composed of foilswhich are stacked one over another and provided with reactant channels.

In this context, a reactant flowing in a first region of the heatexchanger is heated and/or vaporized by a heat-transfer medium whichflows in a second region of the heat exchanger that is inheat-conductive contact with the first region.

German Patent DE 196 39 150 C2 discloses a central heating device inwhich a catalytic oxidation of a fuel takes place. The thermal energywhich is produced in the process can then, for example, be supplied tothe heat carrier and thus be used for operating the above mentioned heatexchanger. The fuel used can be an arbitrary gaseous or liquid mediumwhich is able to be catalytically oxidized. The examples given for thefuel in the above mentioned document are the starting material for thegas generation, in this case methanol, the product gas produced in thegas generation system or a hydrogen-containing exhaust gas of the fuelcell.

When a vaporizer of that kind is now used in a corresponding gasgeneration system for supply to a fuel cell system, then a very poordynamic response ensues, in particular, in the case of step changes inload, because the vaporizer constitutes the first member in the reactionchain, while in the heating region the thermal energy is usuallyproduced with the exhaust gas of the fuel cell and/or of the gasgeneration system, the vaporizer can respond to the required load changeonly in a lagging manner.

Located between the vaporizer and the fuel cell, which ultimatelygenerates the requested power, are several reaction spaces of the gasgeneration system which each have different response times due to theirstructure and which, together with the fuel cell, deliver the exhaustgases required for producing the thermal energy. Due to this feedbackbetween the vaporization region and the combustion region, therefore,the very unsatisfactory dynamic response of the overall system ensues,which turns out to be very disadvantageous for a mobile use of the fuelcell system in a motor vehicle.

SUMMARY OF THE INVENTION

A method of this type is known from European Patent Application EP 0 920064 A1. In this context, methanol and water are vaporized or superheatedin a reforming reactor by thermal contact with a hot gas. The hot gas isproduced in a catalytic burner which receives anode exhaust gas and air.During certain operating phases, in addition, liquid methanol isintroduced into the air supply of the burner. To vaporize thisadditional fuel, a heat exchanger is located in the exhaust area of theburner, the heat exchanger being used to transfer thermal energy fromthe hot exhaust gas of the burner to the air/methanol flow, therebyvaporizing the methanol. This system has the disadvantage that theenergy required for vaporizing the additional fuel is extracted from thehot exhaust gas and is therefore no longer available for heating thereforming reactor.

Therefore, an object of the present invention is to devise an improvedmethod for vaporizing and/or superheating a fuel or a fuel/water mixturefor supply to a gas generation system of a fuel cell system, whereby avery fast dynamic response can be achieved.

Because a part of the fuel itself is metered into the volume flow of theoxygen-containing gas and of the exhaust gases of the fuel cell and/orof the gas generation system, it being possible for theoxygen-containing gas to be, in particular, air but alsooxygen-containing cathode exhaust gas of the fuel cell or a mixture ofboth, the thermal energy content of the volume flow can be increasedwith a fast dynamic response, resulting in the advantage that thecatalytic combustion is also very quickly able to supply a higherthermal energy. This permits at least nearly undelayed vaporizationand/or superheating of the fuel or of the fuel/water mixture, making itpossible to respond very quickly to an increased power demand. Thus, thequantity of vaporized and/or superheated fuel or fuel/water mixturerequired for generating the requested increased power is available tothe gas generating system in a particularly advantageous manner withoutsignificant time delay.

A further advantage arises because the metering, which can be carriedout as a function of the required amount of heat and consequently of theelectric power requested from the fuel cell system, takes place prior tothe actual catalytic combustion and therefore no further thermal energyis extracted from the combustion or from the space, in which thecombustion takes place, for the fine dispersion and/or for thevaporization of the added additional fuel.

In a particularly advantageous embodiment of the present invention, thefuel is atomized when it is introduced into the volume flow of theoxygen-containing gas and the exhaust gases of the fuel cell and/or ofthe gas generation system. In this context, the energy required foratomization can be taken as mechanical energy from the pressure or theflow velocity of the volume flow and of the fuel to be introduced. Afuel, which is dispersed in fine particles in the volume flow, is infact sufficient to ensure the functioning of the catalytic combustion.

In the above mentioned embodiment of the present invention, the devicefeatures a gas-assisted nozzle for this which increases the flowvelocity in the volume flow by a narrowing and introduces the fuel intothe volume flow in this region. In the process, the fuel is atomized inthe volume flow by the energy of flow thereof.

Thus, the reaction chamber for the catalytic combustion receives amixture which is composed of the required gases and the additional,finely dispersed fuel, and is able to be catalytically burned withoutfurther significant absorption of thermal energy. This results in theadvantage that all thermal energy developing during the combustion isimmediately available for vaporizing and/or superheating the fuel or thefuel/water mixture, and that the fuel cell system is therefore veryquickly able to respond to a required step change in load.

BRIEF DESCRIPTION OF THE DRAWING

Further advantageous embodiments of the present invention as well aspossible devices for carrying out the method according to the presentinvention follow from the exemplary embodiment which is schematicallydescribed below with reference to the drawing which shows deviceaccording to the present invention.

DETAILED DESCRIPTION

The only attached FIGURE shows such a heat exchanger 1 which is joinedup by a schematically indicated gas generation system 2 and a fuel cell3. In this context, heat exchanger 1 features two regions 1 a, 1 b whichare in heat-conductive contact with each other but sealed from eachother for the fluids which flow therethrough, respectively.

In this context, region 1 b of heat exchanger 1 has a schematicallyindicated catalytic coating 4, a catalytic filling, or the like, whichis required for a catalytic combustion of the supplied substances. Theenergy which develops during the catalytic combustion in region 1 b ofheat exchanger 1 then reaches the other region 1 a of heat exchanger 1.A liquid fuel or a liquid fuel/water mixture, which, in the exemplaryembodiment shown, is assumed to be a methanol/water mixture (CH₃OH+H₂O),and which is fed to region 1 a, is vaporized in region 1 a of heatexchanger 1 by the thermal energy coming from the catalytic combustion.

Given sufficient thermal energy, the vaporous or gaseous mixture ofwater and methanol is also superheated in region 1 a of heat exchanger1. Then, this mixture of water and methanol gets into gas generationsystem 2, which is indicated schematically and whose mode of operationis known per se, and into fuel cell 3, possibly after an optional gascleaning stage (not shown).

Since the mode of operation of these components 2, 3 is known per se andof no further importance to the present invention, it will not befurther discussed in detail.

Fuel cell 3 then delivers the requested power P in the form of electricpower. Exhaust gases containing combustible residuals such as residualmethanol or residual hydrogen arise in both components 2, 3,predominantly however in an anode chamber of fuel cell 3. According tothe arrows shown in broken lines, these exhaust gases, at leastpartially, reach a line 5 via which they can be fed to region 1 b ofheat exchanger 1 again for catalytic combustion.

If now a higher power P is suddenly requested from fuel cell 3, then, ofcourse, the quantity of methanol/water mixture which is vaporized inheat exchanger 1 has to be increased as fast as possible.

In this context, in order for the entire system to function smoothly, itis required that this additionally added quantity of methanol/watermixture be vaporized in an at least nearly undelayed manner.

However, the problem now arises that mainly the exhaust gases of fuelcell 3 and of gas generation system 2 are used for producing the thermalenergy for the vaporization. However, since at the time of the increasedpower demand, the quantity of exhaust gas has not yet been increasedbecause no increased quantity of vapor is available yet, this feedbackresults in a time delay in the vaporization of the methanol/watermixture and, consequently, in a time delay between the power request andthe actually possible delivery of requested power P by fuel cell 3.

This problem is solved in that liquid methanol fuel (CH₃OH) is fed via aline element 6 to the volume flow of the exhaust gases of gas generationsystem 2 and/or of fuel cell 3.

To improve the combustion, these exhaust gases have previously alreadybeen mixed with an oxygen-containing gas (O₂), here, in particular, airor oxygen-containing exhaust gas of an anode chamber of fuel cell 3,which flows in via a connection element 7. This volume flow now takes upthe fuel from line element 6 which is able to be metered in as afunction of requested power P of fuel cell 3. The mixture formed in thismanner flows into region 1 b of heat exchanger 1 and is catalyticallyburned there.

In this context, it is important that region 1 b already receive amixture of the exhaust gases, the air and the methanol, that has beenuniformly distributed so that no thermal energy, which could be used forvaporizing the methanol/water mixture in region 1 b of heat exchanger 1is extracted in region 1 b of heat exchanger 1 for mixing and/orvaporizing the individual components in the volume flow.

Therefore, in order for the liquid methanol supplied via line element 6to be distributed in the volume flow as uniformly as possible, use ismade of a gas-assisted nozzle 8 which uses the energy of flow of thevolume flow for atomizing the liquid methanol.

To this end, the region, in which line element 6 opens out into line 5,features a narrowing 9 which, due to the law of continuity, causes anaccelerated flow, i.e., a higher flow velocity in the region ofnarrowing 9. In this context, the methanol supplied via line element 6is taken up by the volume flow flowing through line element 6 andatomized therein.

In this connection, narrowing 9 can be designed in the manner of aventuri nozzle, as is schematically shown in the exemplary embodiment,but can also be formed by a lance-type end of line element 6 (not shown)that reaches into line 5.

In this context, the methanol can be metered via a throttle device 10 inline element 6 or via a suitable controllable delivery device (notshown). In this context, the volume flow of liquid methanol flowingthrough line element 6 is in each case controlled in open or closed loopas a function of power P requested from fuel cell 3.

Something comparable to what happens to the liquid methanol is also truefor the air which reaches line 5 via connection element 7. Here too, anarrowing 9′ can optionally be formed which is indicated by a brokenline in the only attached FIGURE. In this context, the mode offunctioning of narrowing 9′ is similar to that of narrowing 9 during thesupply of the liquid methanol only that in the case of narrowing 9′, twogaseous media are mixed with each other. In this process, the air, whichis fed to line 5 via connection element 7, can also be controlled inopen or closed loop as a function of the power requested from fuel 3and, consequently, of the thermal energy necessary in heat exchanger 1.In this context, it is thus possible to use a throttle valve 11 or thelike in connection element 7 to be able to influence the volume flow ofthe air.

1. A device for vaporizing and/or superheating a liquid fuel for a gasgeneration system of a fuel cell system having a fuel cell, the fuelcell or gas generation system producing exhaust gases, the devicecomprising: a heat exchanger having a first region and a second region,the first and the second region being separated from each other andbeing in heat-conductive contact with each other, and a liquid fuelbeing vaporized and/or superheated in the first region, the secondregion containing a catalyst for catalytic combustion; a line forsupplying the exhaust gases to the second region; a line element forsupplying additional liquid fuel to the line; and a connection elementfor supplying oxygen-containing gas to the line; the line, upstream ofthe heat exchanger in a flow direction of the exhaust gases having anarrowing in a region of the line element, and the narrowing beingdownstream of the connection element.
 2. The device as recited in claim1 wherein the line element includes a device for open- or closed-loopcontrol of the volume flow of the additional liquid fuel that flowstherethrough.
 3. The device as recited in claim 1 wherein the narrowingis a venturi nozzle.
 4. The device as recited in claim 1 wherein theline element extends into the line in the region of the narrowing. 5.The device as recited in claim 1 wherein the connection element includesa throttle device.
 6. The device as recited in claim 1 wherein the linehas a further narrowing and the connection element connects to the linein a region of the further narrowing.